In the technical field of optical fiber preforms, there are certain constraints that it is essential to satisfy. Preforms are now being made with greater and greater diameters, e.g. by the following manufacturing methods: plasma outside inside deposition (POID), furnace chemical vapor deposition (FCVD), and vapor axial deposition (VAD). Certain applications require preforms to be made having a core index that is very high (of the order of 50.times.10.sup.-3 ; e.g. for manufacturing optical amplification fibers.
The constraint relating to the (refractive) index of the core of a preform applies to laser amplifiers, whereas the constraint relating to the diameter of a preform applies to the fiber-drawing capacity thereof, i.e. to the length of fiber that can be obtained in a single piece from a given preform, which capacity is at present of the order of 200 km and is likely to be considerably increased in a relatively near future given the necessity of reducing the number of splices in undersea cables between two repeaters, for example.
The only measuring apparatus of the type mentioned above that is available on the market and that is capable of satisfying the needs of users, at least in part, is covered by several patents, such as EP-A2-0377818, WO90/05904, and WO-91/17425. Unfortunately, that known apparatus suffers from several drawbacks, in particular with respect to:
1) price, which is very high and which is not within the reach of most laboratories in which may need to determine the index profile of an optical fiber preform; PA1 2) the method of use, which requires the preform to be driven with motion relative to the measurement cell, which can have the consequence of the preform being damaged due to accidental mishandling, thus requiring it to be discarded and replaced by another, with a considerable loss of income when it is understood that the present cost of optical fiber is about 1 franc per meter; PA1 3) the transparent liquid medium used for optical continuity around the preform, i.e. for ensuring index isotropy, comes into direct contact with the preform, thereby giving rise to severe problems of sealing and of cleaning in the event of the liquid leaking, said liquid having the consistency of an oil; PA1 4) preform diameter lies in a relatively restricted range of values, such that a relatively large number of measurement cells need to be used, with each cell accepting only those preforms whose diameters lie within a range between two end values that are extremely close together; PA1 5) the measurable variation in preform index, .DELTA.n, is less than 40.times.10.sup.-3, whereas there is a requirement to achieve values in the following range: EQU -10.sup.-2 &lt;.DELTA.n&gt;+5.times.10.sup.-2 PA1 6) preform length is limited by size constraints; and PA1 7) the constraints relating to alignment of the preform relative to the measurement cell, which constraints are very severe. PA1 preform support means; PA1 emission means for emitting a light beam that is to scan a cross-section of the preform along a diameter thereof; PA1 an index continuity cell comprising an enclosure provided with a through opening, and serving to press the enclosure around a peripheral annular zone of the preform, together with a cavity formed inside the enclosure so as to surround the above-specified envelope and so as to be in optical communication therewith; PA1 a medium that is deformable and transparent for the light beam being contained inside the cavity of the enclosure and providing optical index continuity, i.e. isotropy, around the preform and relative to the index of the envelope thereof, this being done by passing through a first optical surface constituting an inlet surface for the light beam that scans the preform diametrically, through said medium, and through a second optical surface which constitutes an outlet surface for the light beam, the inlet and outlet first and second optical surfaces being provided in the enclosure transversely relative to the longitudinal axis of the preform, their transverse size being at least equal to the diameter of the preform: PA1 scanning means for diametrically scanning the light beam across the cross-section of the preform along the diameter thereof; PA1 position determining means for determining the position of the incident light beam on the preform relative to the index continuity cell; PA1 receiver and measurement means for receiving the light beam transmitted through the preform, together with deflection, relative to each incidence point of the emitted beam, said receiver and measurement means also serving to measure the deflection of the beam transmitted through the preform relative to each incidence point, and delivering a signal that is a function of said deflection; PA1 means for processing each of the signals delivered by the receiver and measurement means and also for calculating the variation in the index of the preform along a diameter of its cross-section relative to the index of said medium, with this being done on the basis of a set of measurements of deflection; PA1 wherein a deformable and transparent separation interface exists between the deformable index isotropy medium and the outside surface of the envelope of the preform, said interface serving to adapt the deformable index isotropy medium to the outside surface of the envelope of the preform at least over an angular zone thereof, which zone corresponds to an optical measurement zone situated on the path of the light beam, said deformable and transparent interface, and said inlet and outlet optical surfaces for the light beam having a refractive index close to the index of said medium. PA1 two relative slide rails between the measurement cell and the index continuity cell, extending transversely relative to the longitudinal axis of the preform; PA1 two transverse guide grooves for the two relative slide rails; and PA1 drive means for driving the measurement cell parallel to the inlet and outlet surfaces of the index continuity cell. PA1 the two relative slide rails between the measurement cell and the index continuity cell are provided on the two inside faces of the two opposite arms of the horseshoe-shaped measurement cell; and PA1 the two guide grooves for said two relative slide rails are formed in two sides of the index continuity cell, which sides extend transversely relative to the longitudinal axis of the preform and correspond respectively to the inlet and outlet surfaces for the light beam for transversely scanning the preform, said inlet and outlet surfaces occupying the bottoms of the guide grooves. PA1 the two relative slide rails between the measurement cell and the index continuity cell are provided on two sides of the index continuity cell, which sides extend transversely relative to the longitudinal axis of the preform and correspond to the inlet and outlet surfaces for the light beam for transversely scanning the preform, said inlet and outlet surfaces occupying the radially outermost sides of the two rails; and PA1 the two guide grooves for the two relative slide rails are provided in the inside faces of the two opposite arms of the horseshoe-shaped measurement cell.
Thus, the technical problem to be solved consists in designing a measurement apparatus of the type mentioned above which satisfies the requirements of the art better than previously known apparatus of the same type seeking to achieve the same aim, and in particular better with respect to the constraints listed above under 1) to 7).